Reciprocating antirotation pump

ABSTRACT

A pump for pumping discrete liquid volumes is provided with an anti-rotation construction to prevent a nut mated to a rotating head screw from rotating when the lead screw is rotated. The anti-rotation construction comprises a set of rails that slideably fit within a mating set of tracks. The pump also can include a variable homing position for a pump piston having at least two different homing positions.

REFERENCE TO RELATED APPLICATION

This application is based on provisional application Ser. No.60/876,413, filed Dec. 21, 2006.

BACKGROUND OF THE INVENTION

This invention relates to a reciprocating pump capable of dispensingdiscrete microliter or milliliter liquid reagents. More particularly,this invention relates to such a pump which converts rotational motionto linear motion while utilizing an antirotational device.

The science and economics of invitro diagnostic (IVD) testing haschanged with developments in assay reagent variation and assay sizeminiaturization. The number of assays performed annually is increasingas population demographics change and the availability of diagnostictesting increases throughout the world. Instrumentation is constantlyadapted to these changing conditions. The reagents are harsher,including increased usage of salt solutions and acid and alkalinesolutions having significant pH ranges. The liquid that pumps must moveis decreasing from milliliters in volume to microliters in volume whileprecision and accuracy requirements remain constant. Lastly, theinstrumentation is required to perform more tests prior to regular orunscheduled maintenance.

Pumps are used to move fluids within IVD instrumentation and to transferreagents from a source reservoir to one or more test sites. As the assayvolume decreases, more precise pumps are required. However, more precisepumps are more expensive to manufacture. Presently, there is an acuteneed for a precise and accurate pump that is also economical.

Piston pumps have been used in fluid dispensing applications for manyyears and four principle areas have consistently contributed tomanufactured cost. They are the motor, the piston, the seal, and theanti-rotation mechanism. In addition, presently available pumps having ahome position for the pump piston to provide a base value for thecomputer processing unit (CPU) which controls piston movement. Pumps areavailable with the piston home position being either in the pistonaspirated position or in the piston retracted position. Accordingly, itwould be desirable to simplify the anti-rotation mechanism by designingeach part to be multi-functional and manufacturable with multipleprocesses including machined and injection molded methods. Themanufacturing process flexibility helps to control costs during theinitial time period prior to high volume production. In addition, itwould be desirable to provide the pump user with a choice for the pistonhome position to provide flexibility in choosing a desired CPU.

SUMMARY OF THE INVENTION

The present invention provides a pump for liquids which utilizes arotary motor capable of rotating clockwise and counterclockwise, areciprocation pump piston and means for converting rotary motion toreciprocating linear motion. A lead screw which mates within a nutportion of a ferrule is rotated by the motor. The nut portion of theferrule moves linearly and drives a piston portion of the pump todeliver discrete volumes of liquid. An antirotational construction isprovided which prevents rotation of the nut portion of the ferrule. Thedispense resolution of a pump, commonly stated as microliters per step,is proportional to the pitch of the lead screw. Since most piston pumpsare built with off the shelf linear actuators, lead screw pitch islimited to those offered by the linear actuator manufacturer. Thisinvention incorporates a standard stepper motor with a custom machinedlead screw that is pressed onto the shaft enabling an infinite range ofdispense resolutions.

The antirotation construction comprises a set of rails that fit intotracks. Either the rails or tracks can be positioned on the nut portionof the ferrule and the mating rails or tracks are positioned on astationary housing for the nut portion of the ferrule. The length of therails and tracks permits the pump piston to reciprocate between a fullyaspirated position and a fully retracted position.

Variations in pump construction allow for additional features. The nutin the ferrule can be slotted and combined with a single or multipleo-rings as springs to provide low-cost mechanical anti-backlashcompensation. In addition, the stepper motor which turns the lead screwcan be made with a shaft protruding on both sides to provide two pumpheads that are controlled simultaneously with one motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exterior isometric view of the pump of this invention.

FIG. 2 is an exploded isometric view of the components of the pump ofFIG. 1.

FIG. 3 is a side cross sectional view of the pump of this invention.

FIG. 4 is a cross section view through the rail and track of the ferruleof this invention.

FIG. 5 is an isometric view of the anti-backlash nut mechanism.

FIG. 6 is an isometric view of a two headed pump on one stepper motor.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In accordance with this invention, a piston reciprocating pump isprovided with a variable homing switch location. The homing switchlocation can be either the fully aspirated position of the reciprocalpiston or the fully retracted position of the reciprocal piston.

Referring to FIGS. 1 and 2, the pump 10 of this invention comprises arotary motor 12 adapted to rotate clockwise or counterclockwise. Thepiston 3 begins to reciprocate when the motor 12 turns the lead screw 16within the nut portion 18 of the ferrule 20. The ferrule 20 is kept fromrotating by the rails 32 and 33 which fit into tracks 28 and 30, which,in turn, causes the piston 22 to move back and forth.

The shaft 14 is attached to lead screw 16 so that when the shaft 14 isrotated, the lead screw 16 is rotated. Lead screw 16 mates with threadednut portion 18 positioned within the ferrule 20. Piston 22 is secured tothe ferrule 20 so that when the nut portion 18 moves linearly duringrotation of screw 16, piston 22 also moves linearly. A wash disk seal 24fits into pump head 26 and provides the sealing function to permitliquid to be pumped in discrete volumes.

The costly components within a pump 10 are the motor 12, the piston 22,the seal 24, and the anti-rotation mechanism described herein. Thisinvention focuses on simplifying the anti-rotation mechanism and homingswitch location by designing each part to be multi-functional andmanufacturable with either machined or molded process.

The antirotation mechanism which prevents rotation of the ferrule 20comprises tracks 28 and 30 on ferrule 20 which mate with rails 32 and 33on inserts 34 and 35 which extend through openings 36 on fixed housing38. The inserts 34 and 35 are fixed to housing 38 such as with screws.It is to be understood that the tracks 28 and 30 also can be located oninserts 34 and 35 while the rails can be located on ferrule 20 toachieve the same antirotational effect. The housing 38 is provided withtwo incubation posts 40 and 42 which mate with an indicator 44 or 46which identifies the home position of the piston for a given CPU.

The ferrule 20 which connects the piston 22 to the lead screw 16enabling the piston 22 to reciprocate, is now a single piece. Thehousing 38 also contains the tracks 28 and 30 that fit into a rail 32 or33 on inserts 34 and 35 to prevent rotation of ferrule 20. The nut halfof the lead screw 16 nut drive mechanism is also a feature on theferrule, making the ferrule multi-functional on three levels. Theferrule 20 holds the piston 32, incorporates the tracks 28 and 30 foranti-rotation, and incorporates the nut 18 for reciprocation. Thiseliminates parts, simplifies construction, allows for machined or moldedconstruction, and lowers manufacturing costs.

The housing 38 which is the most costly component to manufacture by themachining process, is also designed for injection molding. The housing38 connects the motor 12 to the pump head 26 and incorporates theferrule 20 and homing switch 48. The ferrule 20 which is universal forall pistons, mates to rails 32 and 33 in the universal body 38 toprevent rotation of the nut 18. The homing switch 48 mounts in a cavityon the side of the housing 38. The multi-functional housing designsimplifies construction, allows for machined or molded construction andlowers manufacturing costs.

Long life is a desirable trait for a piston pump. An important componentto long life is material selection. The ferrule 20 slides onto the rails32 and 33 and the lead screw 16 rotates within the ferrule 20. Thesefriction points can wear and contribute to short pump life.

In one embodiment of the invention, the rails 32 and 33 are made ofDelrin AF available from DuPont, which is generically acetal with apolytetrafluoroethylene (PTFE) additive and the ferrule 20 is made frompolyetheretherketone (PEEK). These surfaces exhibit good wearcharacteristics and persons skilled in the art would recognize that manycombinations of materials would also perform satisfactorily. Grease iscommonly applied to these surfaces.

The pitch of the lead screw 16 is typically either 20 or 40 threads perinch and persons skilled in the art would also recognize that a varietyof pitches ranging from 2 through 100 threads per inch are possible.

In addition, persons skilled in the art would recognize that all theparts in the pump could be manufactured with a variety of processesincluding, but not limited to, machinery, plastic or metal injectionmolding, rolling, stamping, or extruding.

Referring to FIG. 3, the pump 10 which includes the lead screw 16 andthe nut portion 18 of the ferrule 20 also includes two switch homingpositions 40 and 42 also includes indicator posts 41 and 42 on homeindicator 48.

Referring to FIG. 4, rails 32 and 33 fit into tracks 28 and 30 offerrule 20.

Referring to FIG. 5, the nut 18 portion of the ferrule 20 is machinedwith four slots, creating four fingers 19 that can flex inward towardthe lead screw 16. When combined with one or more o-rings 21 thatcompress the fingers 19 into the lead screw 16, backlash is eliminatedbetween the lead screw 16 and the nut 18. Persons skilled in the artwould also recognize other anti-backlash methods may be incorporatedinto the ferrule 20 to eliminate backlash and that other spring methodsmay be employed. In addition, the ferrule 20 may be manufactured bymachining, molding, or other methods and may incorporate between two andtwelve fingers 19.

Referring to FIG. 6, the motor 12 may be built with shafts 14 protrudingfrom both sides of the motor 12. Some pump applications require twoliquids to be pumped in different volumes at the same time. This allowsone motor 12 to drive two pump heads 26 for that application. Personsskilled in the art would also recognize the lead screw 16 on either sideof the motor 12 could be different, creating different dispense andaspirate profile curves. For example, one lead screw 16 could be a lefthand thread while the other could be right hand thread. This would makeboth pump heads 26 dispense and aspirate simultaneously. In anotherexample, both lead screws could be right hand thread. This would makeone pump head 26 aspirate while the other dispenses and vice versa whenthe motor 12 direction is reversed.

1. A pump adapted to deliver a plurality of discrete liquid samplescomprising a motor adapted to rotate a shaft including a lead screwclockwise or counter clockwise, a nut mated to said lead screw adaptedto reciprocate when said lead screw is rotated, a first housing for saidnut having an outside surface provided with either a first set of railsor a first set of tracks, a second set of tracks positioned on astationary housing positioned to mate with said first set of rails or asecond set of rails positioned on said stationary housing to mate withsaid first set of tracks, a pump piston mounted on said first housing,said first set of tracks or said second set of tracks having a lengthsufficient to permit said piston to travel a desired distance when saidmotor is rotated.
 2. The pump of claim 1 wherein said first housingincludes said first set of rails.
 3. The pump of claim 1 wherein saidfirst housing includes said first set of tracks.
 4. The pump of claim 3wherein said second set of tracks are positioned on removable insertsthat are attachable to said stationary housing.
 5. The pump of claim 2wherein said second set of tracks is positioned on an inner surface ofsaid stationary housing.
 6. A pump adapted to deliver a plurality ofdiscrete liquid samples and having a variable homing position foraccepting a visible homing flag in one of at least two positions.
 7. Thepump of claim 6 having two of said positions for said homing flag. 8.The pump in claim 2 having an anti-backlash nut feature integrated intothe ferrule.
 9. The pump in claim 2 having two heads actuated by a dualshaft motor with shafts protruding from both sides of the motor.